problem with System()

[AlvaroS]

Hello everyone:

First of all thanks a lot for reading this post and being able to help.
I have a problem whit System command.

if I write:

sprintf(command, "%s %0.0f %0.0f 0 %0.0f %0.0f %s 0 0 +", princi,wps.front().x(), wps.front().y(),wps.back().x(),wps.back().y(),name_bmp);

chdir("/home/"); //To change directory
system(command);

It give me segmentation fault when it runs system(command)

However if I write:

system("./princi 0 747.37 427.68 0 844.37 42.54 prueba_princi.bmp 0 0 +")

it runs perfectly....

What am I doing wrong?
Thanks!

[SGaist]

Hi,

What does it have to do with Qt ?

In any case, check the content of your command string.

Note that you can use QProcess for that kind of task.

[AlvaroS]

@SGaist Yes, command string is right... but I do not why segmentation fault...
I am going to check QProcess

[Joel Bodenmann]

This is not really an answer to your question but still related: You should consider using the QProcess class. This is not only a lot more flexible & portable than the system() function but also integrates well into the Qt framework (signal&slots, properties, ...).
Usually you want to use Qt library components wherever possible.

To still be somewhat useful: Did you check that command is large enough and that the string assembled by sprintf() is correct?

Edit: @SGaist played ninja again....

[mrjj]

@AlvaroS said:

sprintf(command, "%s %0.0f %0.0f 0 %0.0f %0.0f %s 0 0 +", princi,wps.front().x(), wps.front().y(),wps.back().x(),wps.back().y(),name_bmp)

What types are
princi
name_bmp

better not be QStrings :)

[AlvaroS]

@mrjj Hello.
They are const char* !

[SGaist]

Then run your application through the debugger.

[AlvaroS]

@SGaist Now I do this:

    QString program = "./home/princi";
    QStringList arguments;

    arguments << "0" << QString::number(wps.front().x()) << QString::number(wps.front().y()) << "0" << QString::number(wps.back().x()) << QString::number(wps.back().y()) << "prueba_princi.bmp" << "0 0 +" ;

    QProcess myProcess;

    myProcess.start(program,arguments);

it compiles good but my application does not run...

[SGaist]

You call it with a path relative to where you main application is started.

./home/princi is equivalent to $PWD/home/princi/ which is likely not what you want, is it ?

[AlvaroS]

@SGaist yes! but it does not run with that code..

[AlvaroS]

@SGaist Now I do this and nothing as well..

    QString program = "princi";
    QStringList arguments;

    arguments << "0" << QString::number(wps.front().x()) << QString::number(wps.front().y()) << "0" << QString::number(wps.back().x()) << QString::number(wps.back().y()) << "prueba_princi.bmp" << "0 0 +" ;

    QString folder = "/home/";

    QProcess *myProcess = new QProcess(this);

    myProcess->setWorkingDirectory(folder);

    myProcess->start(program,arguments);

[SGaist]

What is the full path to princi ? And what is the full path to your application ?

[AlvaroS]

@SGaist Full path of princi (which is the executable) is /home/
and full path of my QT aplication is /home/workspace

[jsulm]

/home is not the full path to your application except it is called home and is located in the root directory.
So, you can start the program in a shell with /home/princi?
Did you try it?
This is quite strange path: in /home you usually have a directory for each user and /home is not writable by normal user.

[AlvaroS]

@jsulm sorry yes, it is in /home/user1/ but it does not run as well...

[mrjj]

@AlvaroS said:

QString program = "princi";

should be

QString program = "/home/user1/princi";

?

[AlvaroS]

@mrjj It works in the same way... :(

[jsulm]

@AlvaroS So, if you enter /home/user1/princi in a terminal window and press enter it does not start? What happens? Any error message?

[mrjj]

@AlvaroS

Ok, you must debug then
try to use
http://doc.qt.io/qt-5/qprocess.html#error
to see what it thinks.

[AlvaroS]

@jsulm In a terminal if I write

./princi 0 747.37 427.68 0 844.37 42.54 prueba_princi.bmp 0 0 +

in /home/user1/ directory it runs good

[jsulm]

@AlvaroS Then try what @mrjj suggested

[AlvaroS]

This post is deleted!

[AlvaroS]

@AlvaroS said:

@jsulm In a terminal if I write

./princi 0 747.37 427.68 0 844.37 42.54 prueba_princi.bmp 0 0 +

in /home/user1/ directory it runs good

@jsulm said:

@AlvaroS Then try what @mrjj suggested

okey, it gives me UnknowError.... http://doc.qt.io/qt-5/qprocess.html#ProcessError-enum

[mrjj]

@AlvaroS

well you can use
qDebug() "qproc: " << myProcess->error();

and check the code in
http://doc.qt.io/qt-5/qprocess.html#ProcessError-enum

[AlvaroS]

@mrjj Yes thanks!! See my last post. Thanks again. It gives me Unknow error...

[mrjj]

@AlvaroS

" This is the default return value of error()."

You do call it AFTER u call
myProcess->start(program,arguments); ?

[AlvaroS]

@mrjj Yes:

    myProcess->start(program, arguments);

    QProcess::ProcessError error = myProcess->error();

[mrjj]

@AlvaroS
hmm really odd then
it should say FailedToStart

Just to be 100% clear.

princi is never run ?

[AlvaroS]

@mrjj said:

@AlvaroS
hmm really odd then
it should say FailedToStart

Just to be 100% clear.

princi is never run ?

okey look.
if I write:

    myProcess->start(program, arguments);

    myProcess->waitForFinished(3000);

    QProcess::ProcessError error = myProcess->error();

Now error says:
QProcess::Crashed The process crashed some time after starting successfully.

[mrjj]

so it sounds like your princi do run but it crashes?

oh. sorry my bad. start is async so calling error right after was
not correct.

waitForFinished fixed that.

[AlvaroS]

@mrjj Yes... I think so...
How should I pass the arguments?

the arguments of princi are:

0 wps.front().x() wps.front().y() 0 wps.back().x() wps.front().y() prueba_princi.bmp 0 0 +

so my QStringList arguments is:

 QStringList arguments;
arguments << "0" << QString::number(wps.front().x()) << QString::number(wps.front().y()) << "0" << QString::number(wps.back().x()) << QString::number(wps.back().y()) << "prueba_princi.bmp" << "0" << "0" << "+" ;

[mrjj]

it seems fine
You should look inside princi for how it
reads it. it must be that part that is crashing ?

[AlvaroS]

@mrjj yes, but it really rare that if I run from terminal princi runs good...

[AlvaroS]

@mrjj And if I write:

system ("./princi 0 5 10 0 7 8 prueba_princi.bmp 0 0 +")
where 5,10,7 and 8 are wps.front,wps.back....
it runs well...

[SGaist]

What is that + sign doing as parameter ?

[AlvaroS]

@SGaist + is just and argument that i have to pass to princi

[mrjj]

@AlvaroS
well you must debug it then.
Try to qDebug inside princi what you read and compare that to what u read for
system ("./princi 0 5 10 0 7 8 prueba_princi.bmp 0 0 +")

Sadly We cannot guess why you crash from the code shown.

Also you dump the arguments to qDebug before u
call start, just to see what u are sending also.

and then take it from there.